Conventionally, glass sheets are subjected to various surface treatments for the purpose of giving the glass sheets properties suitable for the intended use. For example, surface treatments are disclosed to densify the surface of glass sheets and thus obtain glass sheets having high mechanical strength such as abrasion resistance. For example, Patent Literature 1 proposes a surface treatment method in which an acidic aqueous solution or vapor generated from the aqueous solution is brought into contact with a glass surface having a temperature equal to or higher than the glass transition temperature so as to obtain a glass having a dense surface structure.